Arc generator for an emission spectrometer

United States Patent u?)

Schayes et al.

[ii] 3,906,291

[45] Sept. 16, 1975

[54] ARC GENERATOR FOR AN EMISSION SPECTROMETER

[75] Inventors: Raymond Georges Schayes; Pol Ambroise Ghislain Joseph Gustin, both of Brussels; Jean Fernand Raymond Schmitz, Ganshoren, all of Belgium

[73] Assignee: U.S. Philips Corporation, New York, N.Y.

[22] Filed: Oct. 1, 1973

[21 ] Appl. No.: 402,181

[30] Foreign Application Priority Data

Sept. 29, 1972 Belgium............... 789436

[52] U.S. Cl.......... 315/171; 307/75; 307/86;

315/172; 315/173; 315/176; 356/86

[51] Int. Cl.²........... H05B 41/22; H05B 41/30

[58] Field of Search........... 315/171, 173, 176, 160,

315/167, 170, 172; 356/86, 98; 307/75, 77, 86, 87

[56] References Cited

UNITED STATES PATENTS

3,309,567 3/1967 Flieder et al........................ 315/176

3,376,470 4/1968 Stone et al..........................315/171

3,749,975 8/1970 Walters............................. 356/86 X

Primary Examiner—James W. Lawrence Assistant Examiner—E. R. La Roche Attorney, Agent, or Firm—Frank R. Trifari (57] ABSTRACT

The invention relates to an arc generator for an emission spectrometer. The generator notably comprises an ignition circuit which renders an ignition electrode conductive, and a low-voltage circuit which produces a discharge in a discharge electrode and which notably comprises a capacitor and a coil, but no resistor.

Claims, 5 Drawing Figures

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PATENTED SEM 61975

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SHEET 1 OF 4

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DETECTION

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CIRCUIT

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PATENTED SEP 161975

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SHEET 2 OF 4

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Fig. 3

PATENTEDSEMSiOTS

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SHEET 3 OF 4

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PATENTED SEP 161975

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SHEET U OF 4

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ARC GENERATOR FOR AN EMISSION SPECTROMETER

The invention relates to an arc generator for an emission spectrometer which can be used for various appli- 5 cations. This generator, referred to hereinafter in general as “source”, serves to generate an electric arc discharge on a discharge electrode such that the sample evaporates and is excited.

The said source comprises two principal circuits: on 10 the one hand, the control circuit which serves to fender the ignition electrode conductive, and on the other hand the low-voltage circuit which serves to maintain the energetic discharge on said electrode.

All known sources comprise a low-voltage circuit in 15 which use is made of a capacitor, a coil and a resistor in a given combination, it being possible to adjust the said circuit elements to the different types of discharge required by the analyses.

The arc generator (source) according to the inven- 20 tion comprises a low-voltage circuit in which only a capacitor and a coil are used. The absence of the dissipative circuit element (the resistor) ensures high efficiency of the low-voltage circuit, with the result that the required power is substantially smaller than the power required for spectrum emission measurements by contemporary sources for the same dissipated energy quantity in the discharge electrode.

The invention will now be described in greater detail with reference to the accompanying drawing in which:

FIG. 1 shows a circuit diagram of a source (arc gen erator) according to the invention.

FIG. 2 shows diagrams of the control circuit and the low-voltage circuit thereof, and

FIGS. 3, 4 and 5 show diagrams of circuit waveforms for a further explanation of the operation.

‘ FIG. 1 shows the circuit diagram of an arc generator according to the invention which is of the modular type, comprising the combination of an ignition unit or ignition circuit 1, a control unit or control circuit 2, and ⁴θ a low-voltage unit or low-voltage circuit 3.

The ignition circuit is used for rendering the ignition electrode conductive and is supplied, via a charging circuit 4, from a source which supplies a direct voltage V&lt;to which is derived by rectification of the supply volt- ⁴⁵age.

Control circuit 2 regroups all external control signals of the generator and provides correct selection and switching of the various analytic parameters. The said control circuit 2 supplies the control pulses in a sequence as required for the operation of the other circuits. In addition, the said control circuit 2 also supervises the operation of the generator. The low-voltage unit 3 comprises the low-voltage discharge circuits and the supply sections of these circuits.

FIG. 2 shows the diagrams of the two principal circuits, the ignition circuit (at the top in FIG. 2) serving to render the ignition electrode conductive, and the low-voltage circuit (at the bottom in FIG. 2) which serves to generate the energetic discharge on said electrode.

The power supply of the ignition circuit is provided by a source which supplies a direct voltage V_friB which is obtained by rectification of the supply voltage. When the arc generator is switched on, an auxiliary voltage source is used which supplies an auxiliary voltage V„_KJwhich is obtained in the same manner. The ignition circuit operates as follows. A capacitor 5 is initially discharged. A first control pulse φΐ is applied to thyristor 6. This pulse &lt;/&gt;l renders the thyristor 6 conductive, with the result that the capacitor 5, the coil 7 and the auxiliary voltage source are connected in series. A sinusoidal current pulse passes through the circuit and charges the capacitor 5 to a voltage which is equal to ^—2 V_auj, if the effect of the losses occurring in the circuit is ignored (see FIG. 311).

The capacitor 5 is disconnected from the circuit by switching off thyristor as soon as the current tends to flow in the other direction; in this case the capacitor 5 is then no longer connected to the auxiliary voltage source. As soon as this has taken place, a control pulse φ2 can be applied to thyristor 8. Because the thyristor 8 is now conductive, a series connection is obtained which is formed by the circuit elements 5, 9 and the trigger voltage source, the capacitor 5 having a negative charge corresponding to —2 V_aua,. This circuit then produces a sinusoidal current pulse by which the capacitor 5 is charged to a voltage which is equal to 2V_irie+ 2V_aKJ. (see FIG. 311) if the circuit losses are ignored.

The voltage across the capacitor 5 is continuously measured by a level-detection circuit 10. This circuit supplies a control pulse as soon as a variable threshold is reached. If the control system is designed such that this control pulse is obtained at V₅ = V_5F, a thyristor 11 becomes conductive, with the result that the thyristor 8 is cut off. The energy still present in coil 9 is recovered by the source which supplies the direct voltage V_(rto.

At the instant at which thyristor 8 is cut off, the voltage at the electrodes of this thyristor has the value: ( Vak)8 ~ Isp — (1 + Λ/m) Vtria, in which m is the ratio ³⁵ between the numbers of turns of the coil 9.

As of this instant, capacitor 5 is charged to a predetermined voltage V_5F and is no longer connected to the charging circuit. A control pulse φ3 can then be applied to the control electrode of thyristor 12. The capacitor 5 is then discharged via thyristor 12 and the primary winding of a high-voltage pulse transformer 13.

The variation of the current through the capacitor 5 flows in accordance with FIG. 4-1. The voltage across a capacitor 14 increases consinusoidally until an auxiliary spark gap 15 breaks down. This corresponds to the first part of the discharging of the capacitor 5 (oscillation circuit formed by the series connection of the dispersion inductance of transformer 13 and the capacitors 5 and 14 with respect to the primary transformer side). After the breakdown of the auxiliary spark gap 15, a high-voltage wave having a steep edge is thus applied to an ignition electrode 16 which is also rendered conductive. Before the discharge of the capacitor 5 terminates, the procedure is as if the secondary side of transformer 13 were short-circuited and a half-period oscillation develops which corresponds to the series connection of the dispersion inductance of the transformer 13 and the capacitor 5. This phenomenon ceases as soon as the current tends to change its direction via the thyristor 12 (disconnection from circuit by way of normal changing-over). At this instant the polarity of the voltage on the ends of the capacitor 5 is inverted with respect to the polarity of the charging voltage V_5F at the beginning, so it becomes —V_5R.

The parameters of the circuit are such that in the case of normal operation the voltage V₅._R is larger than V ^v aux·

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The cycle can be repeated by first applying a control pulse φΐ to the control electrode of thyristor 6. This pulse has no effect because said thyristor 6 has the inverted polarisation voltage at this instant. The operation is subsequently continued in the described manner. However, a difference exists in that the initial voltage of the capacitor 5 is no longer equal to —2 V_ni„, but is equal to —V_SH.

In accordance with the desired type of discharge, the low-voltage circuit can function in two different ways, i.e, one-pole operation or oscillating operation. The change-over is effected by means of a contactor 28. One-pole discharging is based on a direct voltage V_BTwhich is derived from the supply voltage by rectification. Using a control pulse φ2, a thyristor 17 is rendered conductive at the same instant as the thyristor 8 of the ignition circuit. It is assumed that at the beginning a capacitor 18 has a negative voltage —E_in which is the case, as will be established hereinafter, after the first cycle. Because the thyristor 17 becomes conductive, the elements 18, 19 and the source supplying the voltage V_BT are connected in series. This is an oscillating circuit in which a sinusoidal current pulse will tend to develop; the duration of this pulse is π V L_ls Ci₈ and the amplitude thereof is given by the expression.

y £.19 (see FIG. 5).

The voltage at the terminals of the capacitor 18 increases according to a cosine function and tends to reach the value 2 V_BT + E_in if the losses in the circuits ate ignored. A voltage V_F is desired on the capacitor 18 for discharging.

To this end it is ensured that the level-detection circuit 25 which continuously measures the voltage at the terminals of the capacitor 18 supplies a control pulse as soon as Κ)₈ = V_F. This is possible in as far as V_B &lt; 2 V_BT 4- E_in.

This condition can be satisfied by a suitable choice of the circuit elements. The control pulse renders a thyristor 20 conductive, which corresponds to the application of a voltage V_BT to the ends of the auxiliary winding of a coil 19. The thyristor 17 is thus cut off if V_F is larger than V_BT (1 + l/m), m being the ratio of the number of turns of the main winding of the coil 19 to the number of turns of the auxiliary winding of said coil. The energy still present in coil 19 is recovered by a thyristor 20 in the source for the voltage V_BT at the instant of disconnection. The capacitor 18 is thus charged to a voltage V_B, and the charging circuit is no longer connected to said capacitor.

Discharging can now commence. Discharging is controlled by the ignition circuit which renders the ignition electrode 16 conductive. The high-voltage pulse having a steep edge is converted into a low-voltage by a lowpass filter (21-22).

The low-voltage ignition circuit is formed by the series combination of a capacitor 18, initially charged to a voltage V_B, a coil 23 and a diode 24. The capacitance of the capacitor 18 and the inductance of the coil 23 are chosen in accordance with the type of discharge required (proper choice of the current peak value and the duration thereof).

The discharge realized is of the one-pole type, while the current is sinusoidal (FIG. 5). The amplitude of this current is equal to and the duration thereof is given by the expression 7Γ V (L₂3 + L₂₁)C₁₈.

The end of discharging is determined by the circuit elements which are not directly coupled to the discharge electrode 16, and hence it is reproducible from one discharge to the other. If the current in the diode 24 tends to change its direction, the discharge current is switched off after one half current period has passed. The polarity of the voltage at the ends of capacitor 18 then changes, and the energy which remains, ignoring the losses in the circuits, is equal to the initial energy ½ C₁₈ V_B² minus the energy dissipated in the ignition electrode 16. This is the previously referred to voltage Ein·

The charging cycle can be resumed as of this negative residual voltage. After the very first discharge the value of Ein is equal to zero, but after a few discharges a residual voltage will always remain.

In order to achieve operation according to the oscillating mode, the change-overs are realized by three sets of contacts 28. The diode 24 is short-circuited, the cathode of thyristor 20 is connected to ground, and the initial direct voltage is increased to the value V_fl7- + V'_BT.

The initial voltage at the ends of the capacitor 18 can then be equal to zero, and charging itself will be performed as previously by rendering thyristor 17 conductive. The voltage at the ends of the capacitor 18 tends to reach the value 2(V_BT + V'_Br), but the charging is interrupted by means of a level-detection circuit 25 if I/₁₈= V_F &lt; 2(K_bt+ V _bt).

As soon as the charging circuit is no longer connected to the capacitor 5, discharging can take place; this discharging will have an oscillating character with a period of 2 π V (L23 + The damping obtained is dependent on the losses incurred in the circuits and of the energy dissipated in the ignition electrode 16. At a given instant, the circumstances prevailing on the terminals of the ignition electrode will be such that after the current has been reduced to zero no further ignition can take place. The said circumstances are dependent on the type of ignition electrode used, of the analysed material, of the atmosphere used, etc.

Consequently, a given uncertainty exists as regards the polarity of the residual voltage which will remain on the ends of capacitor 18 after the last half period of the discharging low-voltage. In order to eliminate this uncertainty, a control pulse is applied to a thyristor 26 before the next charge is started (&lt;/&gt;l in synchronism with the pulse applied to thyristor 6 of the ignition circuit).

Thyristor 26 is connected in series with a coil 27, a part of the low-voltage coil 23, and capacitor 18. If the residual voltage of capacitor 18 is positive, the thyristor 26 becomes conductive, and this conductive state allows passage of one half period of a current whose amplitude is given by

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the duration thereof being equal to

3,906,291 'Ί Cie(L2T+L23+L23&gt; )

As a result, the polarisation of the voltage of the capacitor 18 is inverted. If the residual voltage is negative before application of a pulse φΐ, nothing happens because the thyristor 26 then has a polarisation voltage of opposite sign. It is to be noted that this is also so in the case of operation according to the non-oscillating mode (mono half period): even though the thyristor 26 receives control pulses, it does not become conductive due to the fact that at this instant the voltage on the thyristor terminals has opposite polarity.

The charging cycle can then be completed as usual. The low-voltage capacitor 18 then already has an initial voltage which is equal to zero which is negative. The choice of the type of discharge is also effected by a suitable choice of the capacitance of the capacitor 18 and of the inductance of the coil 23.